This invention pertains to corrosion resistant water dispersed copolymers and more particularly to emulsion copolymers containing minor amounts of epoxy phosphate. The polymeric blend is particularly useful as paint binders in protective surface coating systems.
Emulsion copolymer systems containing phosphorus additives are suggested in U.S. Pat. No. 4,169,086 where the use of organic phosphates and phosphonates are used as buffers in the copolymerization process. Water based coating compositions for use as an interior can liner are suggested in U.S. Pat. No. 3,991,216 wherein the polymer comprises an interpolymer of copolymerized acrylamide monomer, carboxylic monomer, and other ethylenically unsaturated monomers. Commonly assigned copending application Ser. No. 468,364 filed Feb. 22, 1983 discloses a self-curing water dispersed emulsion copolymer blended with an epoxy-acrylic copolymer and a phosphated epoxy copolymer.
It now has been found that a self-curing emulsion copolymer produced by copolymerizing a certain combination of monomers in the presence of a minor amount of water dispersed high molecular weight epoxy phosphate additive used as an emulsifier provides a copolymer particularly useful in binder systems for corrosion resistant protective coatings. The phosphate esters of relatively high molecular weight provide functional groups in conjunction with the self-curing emulsion copolymer which can be further cross-linked with an external cross-linker such as melamine to provide excellent film barrier properties along with other film integrity properties. The self-curing emulsion copolymer comprises copolymerized ethylenic monomers including alkylol acrylamide monomers and carboxyl, hydroxyl, amine, or amide functional monomers. In accordance with this invention, the emulsified or water dispersed epoxy phosphate is used as the primary surfactant for the preparation of the emulsion copolymers. The self-curing water dispersed copolymer can be prepared in water in the presence of a high molecular weight epoxy phosphate to provide a self-curing water dispersed copolymer in combination with a phosphated polymer. The epoxy phosphate esters are prepared by reacting phosphoric acid with relatively high molecular weight adducts of bisphenol A reacted with low molecular weight epoxy resins such as DER 333. The high molecular weight epoxy can be reacted with between about 0.05% and 5% by weight of phosphoric acid by methods suggested in U.S. Pat. No. 4,289,812 and U.S. Pat. No. 4,316,922. These epoxy phosphates are believed to be coreactive with functional groups on the self-curing water codispersed polymer and crosslink with aminoplast crosslinker to form thermoset films with good barrier properties. During the polymerization process, the epoxy phosphate acts as primary surfactant to control the particle size of the self-curing copolymer. The epoxy phosphate also advantageously stabilizes these polymer particles as their diameter increases. During cure, it is believed the epoxy phosphate acts as an acid crosslinking catalyst by promoting reaction between aminoplast external crosslinker and the self-curing copolymeric hydroxyl groups. (The epoxy phosphate may also participate in crosslink formation reactions through reaction of hydroxyl group adjacent to the phosphate.) Utilizing the epoxy phosphate for emulsion copolymerization inparts excellent corrosion resistance to resultant self-curing emulsion copolymers. The copolymer is particularly useful when applied as protective coating for interior surfaces of food and beverage cans by exhibiting excellent resistance to the corrosive effects of soft drinks and certain foods. These and other advantages will become more apparent by referring to the detailed description of the invention and the accompanying illustrative examples.